I've searched around various places and stitched together a file for /etc/X11/xorg.conf. However, using this file gives me only console log-on - if I remove the xorg.conf, I get gdm starting up, but the screen is essentially duplicated on both monitors.

Any idea where I've gone wrong? I've copied in xorg.conf below, the Xorg.0.log is here.

You are currently using radeonhd 1.2.1 which just doesn't support your GPU. According to the X.Org radeonhd wiki page (http://wiki.x.org/wiki/radeonhd), RV770 support was introduced in 1.2.2. If possible, I would suggest upgrading the driver.

You are currently using radeonhd 1.2.1 which just doesn't support your GPU. According to the X.Org radeonhd wiki page (http://wiki.x.org/wiki/radeonhd), RV770 support was introduced in 1.2.2. If possible, I would suggest upgrading the driver.